On Wed, 25 Nov 2015, Jakub Jelinek wrote:
On Wed, Nov 25, 2015 at 09:58:15AM +0100, Marc Glisse wrote:
I guess it got lost in my text, but if a user writes:
unsigned diff = a - b;
if (b > a) { /* overflow */ ... }
else { ... }
Your patch will not detect it. It seems that replacing x-y>x with y>x could
Sorry, already committed the patch (without incremental that hasn't been
tested anyway).
Sorry, I never meant to imply that your patch was wrong in any way or
should be blocked, I like it. I was only discussing possible future
improvements...
It is true that the patch does not detect this, but it is harder that way.
What if it is
if (b > a) ...
// Huge amount of code
r = a - b;
? Trying to emit the subtraction above the comparison would then very
likely increase register preassure.
The same is true whether we write it b > a or (a - b) > a (I don't think
PRE + SCCVN avoid increasing register pressure).
So, I'd really prefer doing x-y>x to y>x only for single use.
Ok (for now).
--
Marc Glisse
